<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<br>
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<h1>结果:<span></span></h1>
<script>
  let i1 = document.querySelector("#i1");
  let i2 = document.querySelector("#i2");
  let s = document.querySelector("span");

  function f(x)
  {
    if(isNaN(i1.value) || isNaN(i2.value)){
      s.innerText = "输入错误"
      return;
    }
    switch (x) {
      case 1:
        s.innerText = i1.value*1 + i2.value*1;
        break;
      case 2:
        s.innerText = i1.value - i2.value;
        break;
      case 3:
        s.innerText = i1.value * i2.value;
        break;
      case 4:
        s.innerText = i1.value / i2.value;
        break;
    }
  }

</script>
</body>
</html>